1. Field of the Invention
The present invention generally relates to processes for removing acidic gases from flue gases, such as the exhaust gases produced by coal and oil-fired utility and industrial plants.
2. Description of the Related Art
Gas-liquid contactors and absorbers, or scrubbers, are widely employed to remove sulfur dioxide (SO2) and acidic gases such as sulfur trioxide (SO3), sulfuric, acid (H2SO4) vapors, hydrochloric acid (HCl) vapors, and hydrofluoric acid (HF) from flue gases produced by utility and industrial plants. Scrubbers generally have a quench zone where a liquid media is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD).
Sulfur dioxide is typically present in flue gases produced by coal and oilboilers at much higher concentrations than HCl, HF and SO3. Removal of SO3 and sulfuric acid vapors from such flue gases helps to reduce a visible plume produced as a result of the formation of a sulfuric acid mist in the quench zone of an FGD systems. The particle size of such a mist is generally in the submicron range, which is sufficiently small to enable the mist to penetrate most FGD scrubbers. Sulfuric acid emissions of as little as about 5 ppmv will usually result in a visible plume. Therefore, it is desirable to remove SO3 from flue gases upstream of the FGD system. During the combustion of coal, most chlorides present in the coal are converted to HCl. The HCl in flue gases is removed very efficiently by SO2 removal systems, and as a consequence can become highly concentrated in the scrubbing solutions. High concentrations of chlorides can interfere with the scrubber efficiency and lead to disposal problems. Therefore, the removal of HCl prior to the FGD system can also be beneficial in certain cases.
As a solution to the above, U.S. Pat. No. 6,126,910 to Wilhelm et al., incorporated herein by reference, teaches the use of soluble sulfite/bisulfite solutions, such as sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3), potassium sulfite (K2SO2SO3$2HOH), potassium bisulfite (KHSO3) and mixtures thereof to remove SO3 and other acidic gases from a flue gas without removing or decreasing the amount of sulfur dioxide also present in the flue gas. The process entails injecting (e.g., spraying) a concentrated solution containing a sulfite/bisulfite into the flue gas stream to react acidic gases (e.g., HCl, HF and/or SO3) and form a reaction product, without reacting the sulfur dioxide. After removal of the acidic gas(es), sulfur dioxide can be removed from the flue gas farther downstream using conventional scrubbing techniques, which can be rendered more technically and/or economically desirable as a result of the absence of SO3. According to Wilhelm et al., a soluble bisulfite salt such as sodium bisulfite selectively removes acidic gases such as HCl, HF and SO3, but will not remove sulfur dioxide. Wilhelm et al. teach that, contrary to the object of the Wilhelm et al. process, reagents such as sodium carbonate (Na2CO3) and lime (CaO) remove sulfur dioxide.
The present invention is a process that enables the use of sodium carbonate (Na2CO3) and sodium bicarbonate (Na2HCO3) solutions to remove SO3 and other acidic gases or vapors from a flue gas without removing or decreasing the amount of sulfur dioxide also present in the flue gas. While sodium carbonate solutions have previously been used to remove SO2 in flue gas desulfurization (FGD) scrubbers, the present invention uses much smaller quantities of solution injected into the flue gas so that SO3 removal is accomplished without substantial amounts of SO2 being removed. For instance, the amount of sodium carbonate/sodium bicarbonate needed is believed to be less than about 10 to 15%, on a molar basis, of the amount of SO 2/H2SO4 present in the flue gas.
An important distinction of this invention is the use of a solution of sodium carbonate/bicarbonate instead of a solid sodium carbonate or sodium bicarbonate material. By introducing the solution as a spray, and by appropriately selecting the spray droplet size and solution concentration, the spray dries on contact with the flue gas and the resulting dry particles of sodium carbonate/bicarbonate are sufficiently small as to provide sufficient surface area to react with SO3 and any H2SO4 and HCl vapors in the flue gas at low concentrations.
Other soluble species believed to be capable of similar use are aqueous solutions of sodium hydroxide (NaOH) and combinations of sodium hydroxide and sodium carbonate solutions. These materials may also be used in combination with the bisulfite solutions taught by Wilhelm et al., particularly solutions of sodium sulfite, sodium bisulfite, potassium sulfite, and/or potassium bisulfite. Solutions of other alkaline salts similar to these species, such as ammonium hydroxide (NH4OH), potassium hydroxide (KOH), potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3), alone or in combination, may also be used in this process for SO3/H2SO4 removal. Specifically, it is believed that a solution containing sodium hydroxide, ammonium hydroxide, potassium hydroxide, potassium carbonate, and/or potassium bicarbonate, with optional additions of sodium sulfite, sodium bisulfite, potassium sulfite, and/or potassium bisulfite, are also capable of being introduced as a spray whose droplet size and solution concentration enable the solution to dry on contact with the flue gas, at which point they become effective to react with SO3,H2SO4 vapors and HCl vapors in the flue gas without removing sulfur dioxide.
The invention finds particular application in coal and oil-fired power plants, and can be used to reduce SO3, vaporous H2SO4 and, to some extent, HCl from flue gases, thereby reducing corrosion and plume opacity problems. If the addition of sodium carbonate and/or sodium bicarbonate solution occurs upstream of the primary particulate control device (e.g., an ESP) of an FGD system, SO3, vaporous H2SO4 and vaporous HCl can be removed along with fly ash while requiring minimal changes to the process. Another option is to add the sodium carbonate/bicarbonate solution upstream of the air preheater section of the FGD. Removal of SO3 at this location will allow the system to be modified to reduce the preheater outlet temperature. Reduction of the gas temperature at this point can result in an increase in the overall boiler efficiency.
Other objects and advantages of this invention will be better appreciated from the following detailed description.